1. Field of the Invention
This invention relates to cableheads for coiled tubing logging operations, the cableheads having mechanical devices for releasing a stuck tool, and more particularly, to a cablehead which allows releasing of a tool when desired while preventing accidental and premature release of the tool.
2. Description of the Prior Art
In heavy-duty logging and/or perforating operations, the logging tool and/or perforating guns may be run into the well using coiled tubing electric line reels. This technique is used particularly often on deviated or horizontal wells. Typically, a cablehead is positioned between the end of the length of coiled tubing and the logging tool and/or perforating guns. The cablehead has a means for mechanically connecting the tubing to the tool or guns and also for providing an electrical connection between a logging cable run down the inside of the coiled tubing and the logging tool or perforating guns. Many of these cableheads also include a means for releasing the tool or guns in the event that the tool or guns becomes stuck in the well.
Prior to the present invention, most cableheads for coiled tubing logging operations have relied on mechanical disconnects to provide a means of releasing in the event of a stuck tool situation. With such a mechanical disconnect, the coiled tubing is generally released from the stuck tool or gun by applying a predetermined amount of tension on the coiled tubing, thereby breaking a set of shear pins in the cablehead. Once the shear pins are broken, the coiled tubing is removed from the well, and the stuck tool or perforating gun may be fished out on a subsequent trip into the well.
A problem with the prior art mechanical disconnect portion of these cableheads is that there is a tendency to accidentally shear during perforating operations. When the guns are shot in wells that are substantially horizontal, this is not much of a problem because the vertical, or axial, shock loading is substantially negligible. However, when a well is deviated at a shallower angle, for example 60.degree., a substantial vertical shock load component is created when the guns are fired. Often, this vertical shock load is enough to prematurely shear the shear pins in the cablehead. Obviously when this happens, the guns are released and left in the well unintentionally.
Another problem with the mechanical disconnect portion of these prior art cableheads is that there are limitations when the shear load for shearing the pins is selected. The natural tendency of a tool operator is to select shear pins with strengths that are very high in order to prevent accidentally releasing the tool or perforating guns when in the well. However, the tensile strength of the coiled tubing is also a factor which must be considered when making the shear pin selection. For example, in a deep well, the weight of the coiled tubing string hanging in the well may be so high that the available over-pull at the surface is limited to a few thousand pounds. If the operator pulls on the tubing string at a higher load than this, there is the risk of parting the tubing at the surface, thereby leaving the entire tubing string and tool in the well which, of course, is a very undesirable situation.
The present invention solves this problem by providing a locking means such as a set of lugs to securely lock the components of the cablehead together so that no loading is prematurely applied to the shear pins. The shear pins may only be sheared after fluid is pumped down the coiled tubing and pressure applied to actuate a piston in the cablehead to release the lugs so that a shearing force may then be applied to the shear pins. Thus, there can be no premature shearing as in prior art mechanical disconnects.